(1) Field of the Invention
The present invention relates to an insulator comprising a rod or pipe made of fiber reinforced plastic (hereinafter referred to as a reinforced plastic rod) and a holding metal fitting to which the rod is secured, and particularly to the improvement of the fatigue life of such an insulator.
(2) Description of the Prior Art
Structures for holding a reinforced plastic rod by a holding metal fitting in an insulator are disclosed, for example, in U.S. Pat. Nos. 3,152,392 and 3,192,622. In the holding structure disclosed in U.S. Pat. No. 3,152,392, a portion of a reinforced plastic rod to be held is inserted into the bore of a sleeve of a holding metal fitting, and the outer circumference of the sleeve is compressed from opposite directions by means of a two-piece polygonal die such that the cross-section of the sleeve is permanently deformed into a polygonal shape to secure the reinforced plastic rod to the holding metal fitting. In the holding structure disclosed in U.S. Pat. No. 3,192,622, a reinforced plastic rod is secured to a holding metal fitting in the following manners. That is, the rod is inserted into the bore of a sleeve having a uniform thickness, and the sleeve is compressed and deformed by means of a tapered polygonal die such that the outer diameter of the sleeve is not substantially reduced at the reinforced plastic rod-receiving tip of the sleeve but is reduced in a large amount at a portion opposed to the rod end; or a reinforced plastic rod is inserted into the bore of a sleeve having a tapered thickness, and the sleeve is compressed and deformed such that the outer diameter of the sleeve is not substantially reduced at the rod-receiving tip of the sleeve but is reduced in a large amount at a portion opposed to the rod end and by means of a polygonal die having opposite planes arranged in parallel. However, these conventional holding structures merely aim to improve the static load performance, and stress concentration occurs in the reinforced plastic rod at the portion opposed to the vicinity of the reinforced plastic rod-receiving tip of the sleeve, and hence the rod is broken at a lower cyclic load.